During the formation of semiconductor devices such as memory devices and microprocessors various layers are formed over a substrate assembly. The substrate assembly can comprise a wafer or wafer section of silicon, gallium arsenide, or other semiconductor, or a semiconductor layer having one or more layers of material formed thereover.
One method for forming a semiconductor devices comprises forming a blanket pad oxide layer over a semiconductor substrate assembly, then forming a blanket nitride layer over the pad oxide layer. The pad oxide and nitride layers are then patterned to expose the substrate assembly. Subsequently, the exposed substrate assembly is oxidized to form a thick oxide layer such as a field oxide and the nitride and pad oxide are stripped. A sacrificial oxide layer is grown over the substrate assembly surface and then stripped. Finally, a thin oxide such as gate oxide is grown over the substrate assembly surface.
During the formation of the field oxide, a nitride region is known to form along the boundary of the field oxide and the substrate assembly (such as the semiconductor substrate). Due to the increased concentration of nitrogen around this region, commonly referred to as "Kooi nitride" or "Kooi ribbon," oxidation is slowed and thinning of any subsequently formed oxide such as gate oxide can result. Gate oxide thinning normally occurs along the interface of the field oxide and the active area. Forming the sacrificial oxide after stripping the pad oxide oxidizes this Kooi nitride and allows for a more uniform oxide layer to form after oxidizing the Kooi nitride.
A process such as that described above can result in undesirable thinning of the field oxide, as shown by the following sample process. During oxidation of the substrate assembly to form a field oxide layer 3000 .ANG. thick, the patterned nitride also slightly oxidizes or forms a "nitride cap." This oxynitride over the nitride must be removed which also removes part of the field oxide, for example 350 .ANG. thereby resulting in a 2650 .ANG. field oxide. The patterned nitride is then removed, for example with H.sub.3 PO.sub.4, which removes the nitride much faster than it removes oxide but a portion of the field oxide is also removed, for example 50 .ANG. to result in a 2600 .ANG. layer of field oxide. The pad oxide must be removed after the nitride strip which again removes a portion of the field oxide, for example 250 .ANG. to result in a 2350 .ANG. layer.
During the growth of a 300 .ANG. sacrificial oxide layer, some thickness is added to the field oxide, for example 100 .ANG. to result in a 2450 .ANG. field oxide, but the added growth is less than the thickness of the sacrificial oxide. The 300 .ANG. sacrificial oxide is then stripped and an overetch is typically included thereby resulting in a 2000 .ANG. field oxide layer. Finally, a gate oxide layer, for example 100 .ANG. is grown which adds, for example, 50 .ANG. to the field oxide thereby resulting in a 2050 .ANG. layer of field oxide.
Thinning the field oxide is undesirable as is known in the art. This is especially true as the distance between transistor gates decreases with improved manufacturing technology because growing a sufficiently thick field oxide between the gates becomes more difficult. A process which reduces thinning of the field oxide would be desirable.